Composite photography is a process for inserting a foreground subject into a background scene. The subject is photographed against a colored backing, typically blue or green. The physical backing often consists of a translucent projection screen, rear illuminated by blue light, thus the name Blue Screen Composite Photography.
In the motion picture industry, the foreground and background scenes are photographed on film which is subsequently scanned to produce high definition RGB signals for computer assisted image compositing. In the compositing process, the colored backing is replaced with a background scene so that the foreground subject and the background scene appear to be a single scene.
A linear image compositing process provides a linear relationship between the luminance and visibility of the colored backing and the signal level of the background scene. It is therefore essential to obtain uniform illumination of the backing. U.S. Pat. Nos. 4,344,085 and 4,625,231 are examples of such a linear compositing process.
Small or rear illuminated backings can be illuminated with good uniformity. A large blue backing that includes a blue floor, and one or more blue set pieces, are front illuminated with multiple light sources which must be kept out of camera view. These multiple light sources; the shadows cast by blue set pieces; the low reflection angle from the blue floor to the camera; all make it impossible to produce a blue field of uniform luminance and color.
The blue floor is necessary if the actor, i.e., the foreground subject, including his feet, is to fully appear in the background scene. Blue set pieces, having the size and shape of objects in the background scene, permit the actor to appear to move among, on, and behind, such background objects. This apparent interaction with background objects adds to the illusion of realism.
The sensitivity of the linear image compositing method to backing luminance and color uniformity, and the need to composite scenes that include a blue floor, led to the invention of "Backing Color and Luminance Nonuniformity Compensation", U.S. Pat. No. 5,032,901. The backing luminance and color correction method taught in this patent requires a registered clear frame for each scene in which all subjects, including all furniture and other non-blue set pieces, have been cleared from camera view. The resulting clear frame shows the blue backing with all its lighting imperfections as well as shadows from blue set pieces.
In accordance with U.S. Pat. No. 5,032,901, correction of nonuniformity is accomplished by selecting a well-illuminated point on the backing and matching the RGB levels for every point on the backing to the RGB levels of the selected reference point. The subject therefore appears before a perfectly uniform field of blue. The clear frame is compared with the uncorrected foreground scene so as to generate an inhibit signal that will prevent the corrections from being applied to the subject.
Backing luminance and color correction as taught by U.S. Pat. No. 5,032,901 is an elegant solution to the backing nonuniformity problem, but its application has been difficult. Motion picture shooting is typically behind schedule, and there is rarely enough time to clear the set of people and set objects, for each scene, to obtain a few clear frames of the backing. Nonuniformity correction has therefore found limited application in motion pictures.
In the case of image compositing for still photography as applied to the graphic arts, the large film formats and cameras are usually not pin registered (i.e., are not accurately locked in position), nor are the camera platforms uniformly stable. Registration of a clear frame under these conditions is difficult and time consuming. Nonuniformity correction has therefore found limited application in the graphic arts.